feat(editor): continue blockquote markers on Enter

Enter at the end of a `> ` line now starts the next line with `> `,
mirroring list continuation. Nested `> > ` keeps its depth, and Enter on
an empty quote drops the marker to exit. Generalises `list_marker` to
`continuation_marker`.

docs/v0.6-markdown.md

@@ -14,6 +14,8 @@ file format.
 
 - [x] Heading lines bolded in render (faux-bold double-strike)
 - [x] List continuation on Enter inside `- ` / `1. ` (with empty-item exit)
+- [x] Blockquote continuation on Enter inside `> ` (nested depth preserved,
+      empty-quote exit); a nested list inside a quote degrades to `> `
 - [x] Soft-wrap at word boundaries
 
 ## Snippets